Recently, with the development of electronic industries and the advance of industrial technologies, various electronic devices are designed toward small size, light weightiness and easy portability. Consequently, these electronic devices can be applied to mobile business, entertainment or leisure purposes whenever or wherever the users are. For example, various image pickup devices are widely used in many kinds of fields such as smart phones, wearable electronic devices, aerial imaging devices or any other appropriate electronic devices. Since the image pickup devices are small and portable, the users can take the image pickup devices to capture images and store the images according to the users' requirements at any time. Moreover, the images can be uploaded to the internet through mobile networks. In other words, these electronic devices not only have important commercial values but also provide more colorful lives to people.
Conventionally, the image pickup device is equipped with an auto focus (AF) system. The auto focus system has an important influence on the imaging quality. Generally, an automatic focusing technology is used for moving a lens module to adjust the distance between the lens module and the object and calculating a focusing evaluation value (e.g., a contrast value) of the image of the object until the maximum focusing evaluation value is searched. In particular, the position of the lens module corresponding to the maximum focusing evaluation value is the position for acquiring the sharpest image. In accordance with a hill-climbing method or a regression method of the conventional automatic focusing technology, one focusing operation needs to continuously move the lens module and search the maximum focusing evaluation value. That is, the implementation of the conventional automatic focusing technology is time-consuming. Moreover, during the process of positioning the lens module, the lens module is possibly shifted to a large extent. Since it is necessary to move the lens module back and forth, the periphery part of the image may be intermittently beyond the sensing region. Under this circumstance, a so-called respiratory motion possibly occurs. The respiratory motion adversely affects the stability of the image focusing process.
From the above discussions, the conventional image pickup device and the conventional image focusing method need to be further improved.